Some formats are supported by Firefox such as YUYV, some others by Chromium such as I420.
My best advice would be using YUY2 that is supported by both.
Chromium would only work with exclusive_caps=1, but Firefox can work with or without.
Try:
sudo rmmod v4l2loopback
sudo modprobe v4l2loopback exclusive_caps=1 video_nr=2
gst-launch-1.0 nvarguscamerasrc ! nvvidconv ! video/x-raw, width=1920, height=1080, framerate=30/1, format=YUY2 ! identity drop-allocation=1 ! v4l2sink device=/dev/video2